The National Zoo would close, but the lions and tigers will get fed; Yellowstone and other national parks will shut down. The Internal Revenue Service could stop issuing refund checks. Customs and Border Patrol agents training officials in Afghanistan might have to come home. And thousands of government-issued BlackBerrys would go silent.

This is what a government shutdown might look like.

With President Barack Obama dismissing a short-term Republican plan to keep the federal government operating past Friday and Speaker John Boehner seeking deeper spending cuts, official Washington braced Tuesday for a replay of the Great Government Shutdowns of 1995 and 1996. For weeks, the Obama administration has been quietly examining the experience of the mid 1990s as a kind of shutdown survival guide. Now those preparations have kicked into high gear.

The White House Office of Management and Budget directed the heads of federal agencies late Monday to share contingency plans with senior managers. On Capitol Hill, the chairman of the House Committee on Administration warned "nonessential employees" Tuesday to turn off their BlackBerrys during a shutdown, or risk punishment for working while on furlough.

And at the Smithsonian Institution, employees were preparing for a lot of disappointed tourists. The 1995 and 1996 shutdowns occurred in winter. Now it is spring break; Linda St. Thomas, a spokeswoman, said the Smithsonian has sold 23,000 advance tickets for cafeteria meals and Imax movies in April.

Her staff was prepared to print "Closed Due to Government Shutdown" signs to tape to windows in museums.

"I got a call yesterday from a woman in Cincinnati; she was bringing a big family, two cars, they were going to drive in from Ohio on Friday," St. Thomas said. "She wanted to know, what should she do? I said, 'I don't know.' "

In any shutdown, the government does not completely cease functioning, of course. Activities that are essential to national security, like military operations, can continue.

Air traffic control and other public safety functions are exempt from shutdowns. Federal prisons still operate; law enforcement and criminal investigations can continue. Employees deemed essential to the functioning of government can come to work. (In ego-driven Washington, a federal shutdown forces high-powered workers to confront their self worth. Many federal officials insisted on showing up in previous shutdowns, apparently unable to come to grips with idea they might not be considered vital.)

During the shutdowns of 1995 and 1996, cleanup work on toxic waste sites was halted because contractors could not be paid and Environmental Protection Agency officials could not monitor cleanup work. Work on more than 3,500 bankruptcy cases was suspended, and the government took a break from going after deadbeat dads. Tens of thousands of passport and visa applications went unprocessed.

Yet in preparation for a possible shutdown of 2011, officials have discovered that the lessons of more than a decade ago are not always relevant. An entire federal agency, the Department of Homeland Security, has come into existence since then. Most of its employees would continue working at airports, borders and seaports as usual, but many managers at headquarters would be temporarily out of work, a federal official said.

The government is much more heavily involved in the mortgage lending market today than in the mid 1990s, and officials are deeply concerned about the economic fallout should the Federal Housing Administration stop guaranteeing loans. Yet Fannie Mae and Freddie Mac, although government-owned, would continue to operate.

Officials are also concerned about unemployment benefits, paid jointly by the federal government and states. In 1995, with the jobless rate at 5.6 percent, states were able to draw on their trust funds to pay the federal portion. Today, many state funds are borrowing from the federal government.

Much would depend on how long a shutdown lasts. (The longest previous episode ran three weeks.) At the federal courts, officials said they could continue operations for 10 days to two weeks, using money from fees paid by people who have filed civil suits.

"After that, who knows?" said Karen E. Redmond, a spokeswoman for the Administrative Office of the United States Courts.

One key question is whether federal employees — the government workforce includes more than 1.9 million civilian workers — would be compensated in the event of a shutdown. Most of those workers are deemed nonessential, but federal officials have not provided an estimate of the number.

John Berry, director of the Office of Personnel Management, posted information on the agency's website Tuesday night saying that "federal agencies do not have the authority to pay their employees during a shutdown." Congress, he said, would decide whether to pay employees who are furloughed.

In the past, Congress often provided such pay, but the political climate now is different, and lawmakers might be less willing to do so.

Members of Congress and White House officials have broad discretion about whether to declare their employees essential. In previous shutdowns, most worked with skeleton crews.

The chairman of the Committee on House Administration, Rep. Dan Lungren, R.-Calif., sent detailed guidance Tuesday to all House members and offices on what they could and could not do during a government shutdown.

A sample letter he provided warned: "Working in any way during a period of furlough, even as a volunteer, is grounds for disciplinary action, up to and including termination of employment. To avoid violating this prohibition, we strongly recommend that you turn your BlackBerrys off for the duration of the furlough."

Administration officials said the Treasury Department would continue one indispensable role: holding regular auctions of federal debt, so the government could borrow more money from the public.

The Office of Management and Budget is not making its contingency plans public, a source of irritation to the American Federation of Government Employees, the union that represents more than 600,000 federal workers. Its president, John Gage, said Tuesday that the union has sued the Obama administration under the Freedom of Information Act to compel the plans' release.

"I think everybody needs to get a better idea of what a shutdown means — who is essential and who is not," Gage said.

Other issues affected by a shutdown

Federal taxes: Just as the income tax deadline approaches, the IRS could be forced to scale down to a skeleton crew. That means tax refunds could be delayed, and the IRS won't be able to help if filers need advice. But you could still face a penalty if you fail to file before the April 18 deadline.

Federal courts: It's unclear how much a government shutdown would affect the federal judiciary system, but bankruptcy and delinquent child support cases may stall. Anything related to protecting citizens or keeping criminals locked up would likely continue, however.

Health services: The Centers for Disease Control and Prevention and the National Institutes of Health were both hit hard by the shutdown in 1995. The NIH stopped taking in new patients, and the CDC halted disease surveillance.

Federal employees/contractors: In the 1995 shutdown, Congress voted to pay back federal employees for the time they were furloughed. It's not clear, in the current political environment, whether lawmakers would make the same decision. Also, in the past 15 years, the federal government has come to rely on contractors for much more of its work. For the thousands of employees who work for government contractors, the situation could be even more uncertain as federal money dries up.

Loan applications: Need to apply for a federal housing loan or a new student loan? The government likely won't be able to process new applications during the shutdown, possibly causing a backlog. And federal agencies also won't have anyone available to answer questions about outstanding federal issues.